Removal of barite weighted mud

ABSTRACT

A composition for dissolving drilling mud including barite particles and a polymer includes an enzyme capable of reacting with the polymer; a chelating agent capable of dissolving the barite particles; and a catalyst capable of promoting a reaction between the chelating agent and the barite particles.

CLAIM OF PRIORITY

This application is a divisional of and claims the benefit of priorityunder 35 USC § 119(e) to U.S. patent application Ser. No. 15/398,989,issued Feb. 18, 2020, which claims priority to U.S. patent applicationSer. No. 62/275,041, filed on Jan. 5, 2016, the entire contents of whichare incorporated here by reference.

BACKGROUND

Drilling mud introduced into an oil or natural gas well forms a filtercake on the drilling formations in the well. The filter cake helps tocontrol the well, for instance, by providing sufficient hydrostatic headto overcome the reservoir pressure exerted by the reservoir of oil ornatural gas accessed by the well. For high pressure formations, heavydrilling mud that contains high density solids can be used. Forinstance, barite can be used as a weighting material in heavy mudintroduced into high pressure formations.

SUMMARY

In a general aspect, a composition for dissolving drilling mud includingbarite particles and a polymer includes an enzyme capable of reactingwith the polymer; a chelating agent capable of dissolving the bariteparticles; and a catalyst capable of promoting a reaction between thechelating agent and the barite particles.

Embodiments can include one or more of the following features.

The polymer includes xanthan gum biopolymer, and in which the enzyme iscapable of degrading the xanthan gum biopolymer. Degraded components ofthe xanthan gum biopolymer are soluble in the composition.

The enzyme includes carbohydrase enzyme.

The chelating agent includes one or more of diethylene triaminepentaacetic acid (DTPA) or ethylenediaminetetraacetic acid (EDTA).

The catalyst includes a salt, such as a carbonate salt.

The composition includes an aqueous solution.

The composition includes an aqueous solution including 10% by weightcarbohydrase enzyme; 10% by weight carbohydrate salt; 20% by weightdiethylene triamine pentaacetic acid; and 60% by weight water.

In a general aspect, a method includes introducing a mud removal fluidinto a drilling well containing drilling mud, the drilling mud includingbarite particles and xanthan gum biopolymer. The mud removal fluidincludes an enzyme capable of reacting with the xanthan gum biopolymer;a chelating agent capable of dissolving the barite particles; and acatalyst capable of promoting a reaction between the chelating agent andthe barite particles. The method includes soaking the mud removal fluidin the drilling well for an amount of time sufficient to dissolve atleast some of the barite particles; and removing the mud removal fluidfrom the drilling well.

Embodiments can include one or more of the following features.

The method includes soaking the mud removal fluid in the drilling wellfor at least 24 hours.

Soaking the mud removal fluid in the drilling well includes mixing,pulsing, or aerating the mud removal fluid in the drilling well.

The drilling well contains a filter cake formed of the drilling mud andin which soaking the mud removal fluid in the drilling well includessoaking the mud removal fluid for an amount of time sufficient todissolve at least some of the filter cake.

The enzyme capable of reacting with the xanthan gum polymer includes anenzyme capable of degrading the xanthan gum biopolymer.

The removed mud removal fluid includes dissolved barite and degradedcomponents of xanthan gum biopolymer.

The enzyme includes carbohydrase enzyme.

The chelating agent includes one or more of diethylene triaminepentaacetic acid (DTPA) or ethylenediaminetetraacetic acid (EDTA).

The catalyst includes a salt, such as a carbonate salt.

The approach described here can have one or more of the followingadvantages. The use of a mud removal fluid including a polymerdissolving component and a barite dissolving component can help removefilter cake formed of barite weighted mud. Removal of barite weightedmud from a well prevents barite particles from being transported throughoil or natural gas flow pipelines, thus reducing erosion and flowfailure that can be caused by the barite particles. Removal of baritefilter cake can also help the well to produce at higher flow rates, thusstimulating oil production.

Other features and advantages are apparent from the followingdescription and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a diagram of a drilling formation.

FIG. 1B is a diagram of barite weighted mud.

FIG. 1C is a diagram of a drilling formation.

FIG. 2 is a flow chart.

FIGS. 3A and 3B are scanning electron microscope (SEM) micrographs offilter cake before treatment with mud removal fluid.

FIGS. 4A and 4B are optical photographs of filter cake before and aftertreatment with mud removal fluid, respectively.

FIGS. 5A and 5B are SEM micrographs of filter cake after treatment withmud removal fluid.

FIGS. 6A and 6B are SEM micrographs of desander solids before treatmentwith mud removal fluid.

FIGS. 7A and 7B are SEM micrographs of desander solids after treatmentwith mud removal fluid.

FIG. 8 is a diagram of a filter cake in a drilling formation.

FIG. 9 is a plot of a target ratio of filter cake solids to mud removalfluid for various filter cake characteristics.

DETAILED DESCRIPTION

This disclosure relates to an approach to removing filter cake formed ofbarite-weighted mud from a well at the conclusion of drillingoperations. Barite-weighted mud is composed of barite particles encasedin a polymer matrix. Barite-weighted mud filter cake is dissolved byexposing the filter cake to a mud removal fluid that includes acomponent capable of dissolving the polymer matrix and a componentcapable of dissolving the barite particles.

Referring to FIG. 1A, a water based drilling mud 200 is introduced intoa well 100, such as an oil well or a natural gas well, during drillingoperations. Drilling mud 200 forms a filter cake 102 on the drillingformation in well 100, thus providing sufficient hydrostatic head toovercome the reservoir pressure exerted by an underground reservoir 104of oil or natural gas and preventing leakage between well 100 andreservoir 104. Following completion of drilling operations, filter cake102 and remaining drilling mud 200 is removed. Failing to remove filtercake 102 or other remaining drilling mud 200 can result in drilling mud200 being transported along oil or natural gas flow paths, which cancause erosion to flow chokes. Removal of barite filter cake can alsohelp the well to produce at higher flow rates, thus stimulatingproduction.

When drilling in high pressure drilling formations, drilling mud 200includes high density solids that act as weighting materials capable ofresisting the high pressure exerted by reservoir 104. For instance,drilling mud 200 that includes barite as a weighting material, sometimesreferred to as barite weighted mud, can be used in high pressuredrilling operations. Referring to FIG. 1B, barite weighted mud 200includes barite particles 202 mixed with mud 204 including a polymer206. For instance, the polymer 206 can be Xanthan gum biopolymer(XC-polymer), which is a high molecular weight polysaccharide that isproduced by fermentation of carbohydrate. XC-polymer is often used indrilling operations, for instance, as a viscosifier for oilfielddrilling, workover, and completion fluids, and as result is often foundin drilling muds.

Removal of barite weighted mud 200 from well 100 is complicated by thepresence of XC-polymer 206 that partially or completely encases bariteparticles 202. In general, XC-polymer 206 cannot be dissolved ordegraded by solvents or acids that are capable of dissolving barite,such as inorganic and organic acids, solvents, esters, oxidizers, andchelating agents. XC-polymer 206 thus acts as a non-degradable barrierthat inhibits contact between the solvent or acid and barite particles202, preventing barite particles 202 from being dissolved. As a result,barite weighted mud 200 cannot be readily dissolved and removed fromwell 100 even using solvents or acids that are capable of dissolvingbarite itself.

Referring also to FIG. 1C, in order to remove barite weighted mud 200from well 100 following drilling operations, mud removal fluid 120 isintroduced into well 100, for instance, after drilling operations areconcluded. Mud removal fluid 120 includes one or more polymer-reactingcomponents that are capable of degrading the polymer component 206 (forinstance, XC-polymer) of the barite weighted mud and one or morebarite-dissolving components that are capable of dissolving the bariteparticles 202. The polymer-reacting component of mud removal fluid 120degrades the XC-polymer 206, exposing the barite particles 202 encasedwithin the XC-polymer 206 to mud removal fluid 120. Thebarite-dissolving component of mud removal fluid 120 can then dissolvebarite particles 202. In some examples, mud removal fluid 120 alsoincludes a catalyst that accelerates or promotes reaction between bariteparticles 202 and the barite-dissolving component of mud removal fluid120.

The polymer-reacting component of mud removal fluid 120 can be anenzyme, such as a carbohydrase enzyme, that is capable of degradingXC-polymer. For instance, the polymer-reacting component can degradeXC-polymer into smaller components that can be suspended or dissolved inthe mud removal fluid 120. The enzyme can be a bare, non-encapsulatedenzyme. Example carbohydrase enzymes include amylase, maltase, sucrose,lactase, or other carbohydrase enzymes. The enzyme can be an enzyme thatfunctions within a certain pH range, such as a pH greater than 8,greater than 9, greater than 10, greater than 11, greater than 12, or inanother range. The enzyme can be activated without a triggering signal.

The barite-dissolving component of mud removal fluid 120 can be achelating agent, such as diethylene triamine pentaacetic acid (DTPA),ethylenediaminetetraacetic acid (EDTA), or another chelating agent, thatis capable of dissolving barite particles 202. The mud removal fluid 120can also include a catalyst that helps the chelating agent to dissolvethe barite, for instance by promoting and accelerating the reactionbetween the chelating agent and barite. Example catalysts can includecarbonate salts such as sodium carbonate or potassium carbonate, sodiumhydroxide, potassium hydroxide, potassium chloride, iron chloride,cesium chloride, or other catalysts.

The mud removal fluid 120 can be an aqueous solution including at leastabout 1% by weight carbohydrase enzyme, such as between about 1% and 15%carbohydrase enzyme or between about 5% and 15% carbohydrase enzyme,such as about 1%, about 2%, about 5%, about 10%, about 15%, or anotheramount. The mud removal fluid can include at least about 1.0% by weightchelating agent, such as between about 5% and 40% chelating agent, suchas about 5%, about 10%, about 15%, about 20%, about 25%, about 30%,about 35%, about 40%, or another amount. The mud removal fluid caninclude at least about 0.5% by weight catalyst, such as between about 5%and 20% catalyst, such as about 5%, about 10%, about 15%, about 20%, oranother amount. A specific example of a mud removal fluid is an aqueoussolution with 10% by weight carbohydrase enzyme, 10% by weight carbonatesalt, and 25% by weight diethylene triamine pentaacetic acid.

Referring still to FIG. 1C, when mud removal fluid 120 is introducedinto well 100, the polymer-reacting component of mud removal fluid 120degrades polymer 206 coating barite particles 202 (FIG. 1B). Thebarite-dissolving component of mud removal fluid 120 can then come intocontact with and dissolve barite particles 202. As the polymer 206 isdegraded and the barite particles 202 are dissolved, the filter cake 102gradually becomes thinner (as shown in FIG. 1C) and in some cases can becompletely removed. Mud removal fluid 120′, including degradedcomponents 110 of the polymer and dissolved barite 112, can be removedfrom well 100. By treating barite weighted mud 200 with mud removalfluid 120, the removal of barite weighted mud 200 from well 100 can beenhanced compared with treatment by ordinary solvents or acids, such asenhanced by a factor of two.

In some cases, a high density solid other than barite can be used as aweighting component in a heavy mud introduced into a high pressuredrilling formation. Example high density solids can include manganesetetra oxide or hematite. In these cases, mud removal fluid 120 caninclude a component capable of dissolving the appropriate high densitysolid.

Referring to FIG. 2, in an example process, barite weighted mud isintroduced into a drilling well (300) and a filter cake is allowed toform on the drilling formation (302) in order to prevent drilling fluidfrom leaking into the drilling formation. Drilling operations areconducted (304). When drilling operations conclude, a mud removal fluidis introduced into the drilling well (306) to dissolve the filter cakeformed on the drilling formation. The mud removal fluid includes anenzyme that is capable of dissolving a polymer coating the bariteparticles in the mud and a component, such as a chelating agent, capableof dissolving the barite itself. The mud removal fluid can also includea catalyst capable of promoting the reaction between the chelating agentand the barite.

The mud removal fluid is allowed to soak in the drilling well for anamount of time sufficient to dissolve some or all of the barite weightedmud forming the filter cake on the drilling formation (308). After thedesired amount of time has elapsed, the mud removal fluid, whichcontains polymer and barite dissolved from the barite weighted mud ofthe filter cake, is extracted from the drilling well (310). Forinstance, the mud removal fluid is allowed to soak in the well for atleast 24 hours, at least 48 hours, at least 72 hours, at least 96 hours,or for another time period. The longer the mud removal fluid is allowedto soak in the formation, the more barite removal will be achieved. Therate of barite removal, and thus the time until complete barite removalis achieved, can depend on factors such as the amount of barite, thetemperature in the well, or other factors. In some examples, the mudremoval fluid can be allowed to soak for extended time periods beyondthe time to achieve removal. For instance, in some operations, the mudremoval fluid can soak for few days or weeks before flowback is planned.

In some examples, the mud removal fluid is allowed to soak passively. Insome examples, the mud removal fluid is actively mixed in the drillingwell, for instance, by physically stirring, pulsing, or agitating themud removal fluid in the drilling well, aerating the mud removal fluidin the drilling well. The pressure or temperature in the drilling wellcan be elevated compared to ambient temperature or pressure. Forinstance, the pressure in the well can be about at least about 1000 psi,such as between 1000 psi and about 10,000 psi, and the temperature inthe drilling well can be about 270° F. During the soaking time, at leastsome of the polymer coating the barite particles in the barite weightedmud is dissolved by the enzyme, thus exposing the barite particles tothe chelating agent and the catalyst in the mud removal fluid. At leastsome of the barite particles in the barite mud are thus also dissolvedduring the soaking time.

EXAMPLES

The efficiency of mud removal fluid at removing various materials,including dry barite, filter cake solids, and desander solids, wasinvestigated.

Example 1 Treatment of Dry Barite

Dry, industrial grade barite was treated with a mud removal fluid toinvestigate the ability of the mud removal fluid to dissolve the barite.The mud removal fluid included 10% diethylene triamine pentaacetic acid(DTPA), 10% carbohydrase enzyme, and 20% potassium carbonate salt in anaqueous solution.

X-ray fluorescence (XRF) and a scanning electron microscopy (SEM) wereused to determine the composition of the dry barite prior to treatmentwith the mud removal fluid. The starting composition of the dry bariteis shown in Table 1.

TABLE 1 Dry barite composition prior to treatment with mud removalfluid. Weight percents of individual elements may not necessarily add upto 100.00 because of rounding and minor imprecision in the method ofanalysis. Element Weight percent Ba 72.03 S 13.12 Na 10.29 Si 2.55 Mg1.37 Fe 0.45 Sr 0.17 Total 100.00

To study the time dependence of barite dissolution, 20 gram (g) samplesof dry barite were soaked in 500 g of mud removal fluid at 270° F. forvarious amounts of time ranging from three hours to 168 hours. After thedesignated amount of time, the remaining material was weighed and thepercentage of solids removed from each sample was calculated. Theresults of these tests are shown in Table 2. The percentage of solidsremoved from the dry barite samples increases with increasing exposuretime to mud removal fluid.

TABLE 2 Effect of exposure time to mud removal fluid on dissolution ofdry barite. Test time Percentage of (hours) barite removed 3 56.31 658.68 12 67.51 24 76.94 48 76.44 72 77.45 96 78.90 120 79.51 168 79.92

To study the effect of the ratio of dry barite to mud removal fluid,barite samples of different weights were each soaked in 40 milliliters(mL) of mud removal fluid at 270° F. for 24 hours. Following thesoaking, the remaining material was weighed and the percentage of solidsremoved from each sample was calculated. The results of these tests areshown in Table 3. A lower ratio of dry barite to mud removal fluidresults in greater removal of dry barite.

TABLE 3 Effect of the ratio of barite to mud removal fluid ondissolution of dry barite. Solid/liquid Removal ratio percentage 1 g/40mL 90.7 2 g/40 mL 69.25 3 g/40 mL 44.9 4 g/40 mL 39.17

Example 2 Treatment of Filter Cake Solids

Wet and dry filter cake solids and actual filter cakes were treated withmud removal fluid to investigate the ability of the mud removal fluid todissolve various types of filter cake solids. The mud removal fluidincluded 10% diethylene triamine pentaacetic acid (DTPA), 10%carbohydrase enzyme, and 20% potassium carbonate salt in an aqueoussolution.

To study the removal of filter cake solids, filter cake drilling mud wascollected from the mud tanks of an actual drilling operation. Theproperties of the mud used in the following examples are given in Table4, where HTHP refers to a high temperature, high pressure cell, and theformulation of the mud is given in Table 5.

TABLE 4 Properties of filter cake drilling mud. Property Value UnitsDensity 120 pounds per cubic foot (lb/ft³) PV 30-40 centipoise (cP) YP24-26 pounds per 100 square feet (lb/100 ft²) 10 sec gel 10-20 lb/100ft² 10 min gel 15-25 lb/100 ft² Filtrate, HTHP @ 18-24 milliliters per30 320° F./500 psi minutes (mL/30 min) HTHP cake 1/32^(nd) -3/32^(nd)inches (in) pH 9-10 MBT active clay 4.0-6.0 pounds per barrel (lb/bbl)Ca⁻ 150,000 milligrams per liter (mg/L)

TABLE 5 Components of filter cake drilling mud. Component Amount UnitsWater 0.691 barrels (bbl) Bentonite 4 pounds (lb) XC-polymer 0.5 lbfiltration control agent 0.25-0.50 lb KCl 20.0 lb KOH 0.5 lb NaCl 66 lbBarite 352.0 lb CaCo₃ medium 5.0 lb Sodium sulfite 0.25-0.30 lbLubricant 1.0-2.0 lb HPHT and dynamic 0.3-0.56 gallons (Gal) fluid losscontrol hydrogen sulfide 2 lb scavenger

To study the removal of wet filter cake solids, filter cake drilling mudcollected from the mud tanks of an actual drilling operation wasfiltered and used directly for solubility tests, without drying. 20 gsamples of wet filter cake solids were soaked in 500 g of mud removalfluid at 270° F. for various time periods ranging from 24 hours to 96hours. After the designated amount of time, the remaining solids weredried and weighed and the percentage of solids removed from each samplewas calculated, based on the measured weight loss of the sample. Theresults of these tests are shown in Table 6. These results indicate thatmaximum removal of barite was achieved within the first 24 hours ofsoaking; soaking times of less than 24 hours may cause less than maximumbarite removal.

TABLE 6 Effect of exposure time to mud removal fluid on dissolution ofwet filter cake solids. Test time Percentage of (hours) solids removed24 76.16 48 77.83 72 78.46 96 77.5

To study the removal of dry filter cake solids, filter cake drilling mudcollected from the mud tanks of an actual drilling operation wasfiltered and dried at 80° C. for a few days. 20 g samples of dry filtercake solids were soaked in 500 g of mud removal fluid at 270° F. forvarious amounts of time ranging from 24 hours to 96 hours. After thedesignated amount of time, the remaining solids were weighed and thepercentage of solids removed from each sample was calculated. Theresults of these tests are shown in Table 7. The mud removal fluid wasmore effective at removing the wet filter cake solids than at removingthe dry filter cake solids. The percentage of dry filter cake solidsremoved by mud removal fluid increases slightly with increasing soakingtime.

TABLE 7 Effect of exposure time to mud removal fluid on dissolution ofdry filter cake solids. Test time Percentage of (hours) solids removed24 69.41 48 71.27 72 73.42 96 74.39

To study the removal of actual filter cake, filter cake drilling mud wascollected from the mud tanks of an actual drilling operation and formedinto filter cakes in high pressure, high temperature (HPHT) cells. 200 gof collected filter cake drilling mud was placed into each HPHT cell andsubjected to a temperature of 270° F. and a pressure of 400 psi tosimulate conditions inside a drilling well for 3 hours to allow filtercake formation on a ceramic disk in the HPHT cell.

XRF was used to determine the composition of the filter cake prior totreatment with mud removal fluid. The starting composition of tworepresentative samples of filter cakes formed in HPHT cells is shown inTable 8. Referring to FIGS. 3A and 3B, SEM micrographs of the two filtercake samples show the presence of a polymer material surrounding solidparticles, which are formed of a mixture of calcium carbonate and bariteparticles. FIG. 4A shows an optical photograph of a filter cake formedin an HPHT cell before treatment with mud removal fluid.

TABLE 8 Filter cake composition prior to treatment with mud removalfluid. Sample 1 Sample 2 Element Weight percent Weight percent O 34.1532.60 Na 2.99 3.17 Mg 2.07 1.83 Al 1.41 1.05 Si 2.81 3.33 S 8.52 9.04 Cl1.68 1.77 K 0.39 0.27 Ca 4.33 3.97 Fe 2.91 0.98 Ba 38.75 41.99 Total100.00 100.00

Following filter cake formation and after draining excess mud from thecell and leaving only the filter cake, 500 g of mud removal fluid wasadded to each HPHT cell and allowed to remain in the cell at 270° F. and400 psi for various amounts of time ranging from 24 hours to 96 hours.After the designated amount of time, the remaining filter cake in eachsample was weighed and the percentage of filter cake removed wascalculated. The results of these tests are shown in Table 9. Thepercentage of filter cake removed by mud removal fluid increases withincreasing time of exposure to the mud removal fluid in the HPHT cells.

TABLE 9 Effect of exposure time to mud removal fluid on dissolution ofactual filter cakes in HPHT cells. Test time Percentage of (hours)solids removed 24 67.38 48 70.85 72 74.63 96 78.44

The composition of the remaining material in two filter cakes treatedfor 48 hours was determined using XRF and is shown in Table 10. Thebarite concentration in the filter cakes was reduced significantly bytreatment with mud removal fluid, from around 40% (Table 8) to less than3%, demonstrating the ability of mud removal fluid to remove barite fromactual filter cakes. In addition, Mg and Ca compounds react with the mudremoval fluid and were removed, and calcium chloride was removed byreaction with the mud removal fluid or by dilution following treatment.

TABLE 10 Filter cake composition following treatment with mud removalfluid at 270° F. and 400 psi for 48 hours. Sample 1 Sample 2 ElementWeight percent Weight percent O 38.40 38.71 Al 3.26 3.46 Si 23.11 24.51S 4.05 3.42 K 8.53 8.50 F 15.34 13.24 Fe 4.92 5.23 Ba 2.40 2.46 Na N/A0.47 Total 100.00 100.00

Referring to FIGS. 5A and 5B, SEM micrographs of the two treated filtercake samples show that the morphology of the material is more distinctthan the filter cakes before treatment (FIGS. 3A and 3B), indicatingthat the both polymer material and barite particles have been removed.Referring also to the optical photograph of FIG. 4B, the treated filtercake sample contains primarily barite particles; calcium carbonate hasbeen dissolved. The treated filter cake sample is visibly smaller andcleaner following treatment with mud removal fluid.

Example 3 Treatment of Desander Solids

Desander solids are solids collected from desanders during well backflow. Wet and dry desander solids collected from a 15 k desander at aflow rate greater than 30 mmscfd were treated with mud removal fluid toinvestigate the ability of the mud removal fluid to dissolve varioustypes of desander solids. The mud removal fluid included 10% diethylenetriamine pentaacetic acid (DTPA), 10% carbohydrase enzyme, and 20%potassium carbonate salt in an aqueous solution.

XRF was used to determine the composition of the desander solids priorto treatment with mud removal fluid. The starting composition of tworepresentative samples of desander solids is shown in Table 11. SEMmicrographs of the two samples of desander solids are shown in FIGS. 6Aand 6B.

TABLE 11 Composition of desander solids prior to treatment with mudremoval fluid. Sample 1 Sample 2 Element Weight percent Weight percent O34.04 36.08 Mg 1.04 0.82 Al 1.12 1.79 Si 6.70 4.87 S 10.87 11.09 K 0.340.63 Ca 1.87 1.72 Fe 1.86 2.09 Ba 42.15 40.90 Total 100.00 100.00

To study the removal of wet desander solids, desander solids were useddirectly for solubility tests, without drying. 20 g samples of wetdesander solids were soaked in 500 g of mud removal fluid at 270° F. forvarious amounts of time ranging from 24 hours to 96 hours. After thedesignated amount of time, the remaining desander solids were weighedand the percentage of solids removed from each sample was calculated.The results of these tests are shown in Table 12. The percentage of wetdesander solids removed by mud removal fluid increases slightly withsoaking time.

TABLE 12 Effect of exposure time to mud removal fluid on dissolution ofwet desander solids. Test time Percentage of (hours) solids removed 2470.95 48 71.54 72 72.18 96 75.68

To study the removal of dry desander solids, desander solids were driedat 80° C. for a few days. 20 g samples of dry desander solids weresoaked in 500 g of mud removal fluid at 270° F. for various amounts oftime ranging from 24 hours to 96 hours. After the designated amount oftime, the remaining desander solids were weighed and the percentage ofsolids removed from each sample was calculated. The results of thesetests are shown in Table 13. The mud removal fluid was more effective atremoving the wet desander solids than at removing the dry desandersolids. The percentage of dry desander solids removed by mud removalfluid increases slightly with increasing soaking time.

TABLE 13 Effect of exposure time to mud removal fluid on dissolution ofdry desander solids. Test time Percentage of (hours) solids removed 2467.42 48 67.94 72 68.22 96 69.44

The composition of two dry desander solid samples following treatmentwith mud removal fluid for 48 hours was determined using XRF and isshown in Table 14. The barite concentration in the desander solids wasreduced significantly by treatment with mud removal fluid, from about40% to less than 2.5%, demonstrating the ability of mud removal fluid toremove barite from desander solids. Referring to FIGS. 7A and 7B, an SEMmicrograph of the first treated dry desander solid sample shows a moredistinct morphology than the non-treated sample (FIG. 6A). FIG. 7B is amagnified SEM micrograph of the sample shown in FIG. 7A, showingdistinct crystal structure within the seemingly amorphous structure ofFIG. 7A.

TABLE 14 Composition of desander solids following treatment with mudremoval fluid. Sample 1 Sample 2 Element Weight percent Weight percent O40.76 54.63 F 11.63 Na 0.48 Mg 0.24 Al 2.80 Si 25.34 44.92 S 4.09 K 7.600.18 Fe 4.62 0.26 Ba 2.46 Total 100.00 100.00

Example 4 Mud Removal Fluid Volume

FIG. 8 is a schematic representation of a filter cake 402 on a wall 404of a drilling formation 400. A fluid 406, such as drilling mud, ispresent within drilling formation 400. A materials balance calculationcan be performed to determine the volume of mud removal fluidappropriate to remove the filter cake 402.

The volume of filter cake 402 can be calculated as follows:

${V_{fk} = {\frac{\pi}{4}{L\left\lbrack {d_{h}^{2} - \left( {d_{h\;} - h_{mc}} \right)^{2}} \right\rbrack}}},$where V_(fk) is the volume of filter cake 402, L is the length ofdrilling formation 400, d_(h) is the diameter of the open hole ofdrilling formation 400, and h_(mc) is twice the thickness of filter cake402 (the factor of two accounting for the presence of filter cake 402 onopposite walls 404 of drilling formation 400).

The weight of filter cake 402 with volume V_(fk) can be calculated asfollows:W _(fk) =V _(fk)ρ_(fk),where ρ_(fk) is the density of the filter cake solids forming filtercake 402.

The volume V_(R) of the open hole in drilling formation 400, and hencethe volume of mud removal fluid that will fill the open hole, can becalculated as follows:

$V_{fl} = {\frac{\pi}{4}{{L\left( {d_{h} - h_{mc}} \right)}^{2}.}}$

The ratio V_(R) of the volume of filter cake (V_(fk)) to the volume ofmud removal fluid (V_(fl)) filling the open hole in drilling formation400 can be simplified to:

$V_{R} = {\frac{d_{h}^{2} - \left( {d_{h} - h_{mc}} \right)^{2}}{\left( {d_{h} - h_{mc}} \right)^{2}}.}$

Filter cake solids generally have a density of between about 2 to 4g/cm³, depending on the weighting material. The ratio of the weight of afilter cake to the volume of mud removal fluid that will fill the openhole is thus given by:

$\frac{W}{V} = {\frac{d_{h}^{2} - {\left( {d_{h} - h_{mc}} \right)^{2}\rho_{fk}}}{\left( {d_{h\;} - h_{mc}} \right)^{2}}.}$

Given the ratio V_(R) of the volume of filter cake (V_(fk)) to thevolume of mud removal fluid (V_(fl)) and the weight to volume ratio of afilter cake, the weight of a filter cake that can be removed by a givenvolume of mud removal fluid can be determined.

Referring to FIG. 9, in a specific example, for a gas well with a holeof diameter d_(h) of 9⅝ inches, the ratio of filter cake weight to mudremoval fluid can be plotted as a function of filter cake thickness forvarious filter cake densities. The drilling mud used in Example 2 has afilter cake thickness of about 2/32^(nd) inches. For this mud, the plotin FIG. 9 indicates that a ratio of 20 g of filter cake solids in 500 mLof mud removal fluid is sufficient, which was the ratio used in thetreatments described in Example 2.

Other implementations are also within the scope of the following claims.

What is claimed is:
 1. A method comprising: introducing a mud removalfluid into a drilling well containing drilling mud, the drilling mudcomprising barite particles and a polysaccharide polymer, the mudremoval fluid comprising: between 10% and 15% by weight of acarbohydrase enzyme capable of reacting with the polysaccharide polymerat a potential of hydrogen (pH) greater than 8; between 5% and 25% byweight of an aminopolycarboxylic acid chelating agent capable ofdissolving the barite particles; and between 5% and 20% by weight of acatalyst capable of promoting a reaction between the aminopolycarboxylicacid chelating agent and the barite particles, wherein the catalyst isselected from the group consisting of sodium hydroxide, potassiumhydroxide, and iron chloride; and soaking the mud removal fluid in thedrilling well for an amount of time sufficient to dissolve at least someof the barite particles; and removing the mud removal fluid from thedrilling well.
 2. The method of claim 1, comprising soaking the mudremoval fluid in the drilling well for at least 24 hours.
 3. The methodof claim 1, in which soaking the mud removal fluid in the drilling wellcomprises mixing, pulsing, or aerating the mud removal fluid in thedrilling well.
 4. The method of claim 1, in which the drilling wellcontains a filter cake formed of the drilling mud and in which soakingthe mud removal fluid in the drilling well comprises soaking the mudremoval fluid for an amount of time sufficient to dissolve at least someof the filter cake.
 5. The method of claim 1, in which the enzymecapable of reacting with the polysaccharide polymer at a potential ofhydrogen (pH) greater than 8 comprises an enzyme capable of degradingthe polysaccharide polymer at a potential of hydrogen (pH) greater than8.
 6. The method of claim 1, in which the removed mud removal fluidcomprises dissolved barite and degraded components of the polysaccharidepolymer.
 7. The method of claim 1, in which the chelating agentcomprises one or more of diethylene triamine pentaacetic acid (DTPA) orethylenediaminetetraacetic acid (EDTA).
 8. The method of claim 1,comprising introducing the mud removal fluid into the drilling wellafter drilling operations in the drilling well are concluded.
 9. Themethod of claim 1, in which the barite particles are at least partiallycoated with the polysaccharide polymer, and in which soaking the mudremoval fluid in the drilling well comprises: degrading thepolysaccharide polymer at a potential of hydrogen (pH) greater than 8 bythe enzyme; and dissolving the barite particles by the chelating agent.10. The method of claim 1, in which introducing the mud removal fluidinto the drilling well comprises introducing an aqueous solutioncomprising the enzyme, the chelating agent, and the catalyst.
 11. Themethod of claim 1, comprising dissolving at least some of the bariteparticles with the mud removal fluid.
 12. The method of claim 1, whereinsoaking the mud removal fluid in the drilling well comprises soaking themud removal in the drilling well at a pH greater than 8.